It is sometimes advantageous to coat a substrate, especially a metal substrate, with a coating having special properties, for example wear-resistance or corrosion-resistance. Wear-resistance can be imparted to a substrate by coating it with a mixture of a hard abrasive material, such as powdered tungsten carbide or the like, and a hard filler metal in which the abrasive material is dispersed. Corrosion resistance can be imparted to a substrate with a similar coating composed of a corrosion-resistant metal or alloy. Other special or improved physical properties can be obtained in a similar manner substituting other matrix materials.
Prior to the work of Breton et al. (U.S. Pat. Nos. 3,743,556, 3,916,506, and 4,194,040 the specifications and claims of which are incorporated herein by reference), it was often difficult to produce the above type of coating, especially on substrates having an intricate or complicated shape, substrates needing a reliable metallurgical bond between substrate and coating, substrates requiring nearly void-free coatings or substrates requiring a coating of greater than 0.020 of an inch. In prior art methods, which used techniques such as plasma and flame spraying and dusting, it was difficult to provide uniform coatings on substrates, and such coatings as were produced were not void-free, nor were the coatings metallurgically bonded to the substrate to prevent spalling in use.
Breton et al. eliminated many of the difficulties that existed in the prior art methods by providing a unique coated article and method of manufacturing it. The method uses a first layer of a desired thickness of a high-melting-point powdered matrix material in an organic binder material and a second layer of a lower-melting-temperature powdered brazing filler material also in an organic binder. The first layer is placed on the substrate with the second layer in turn placed on the first layer. The matrix is characterized as being wettable by the braze filler metal or alloy in the molten state. The layered assembly is then heated to decompose the binder and melt the filler metal or alloy which is infused by capillary action into the matrix layer. Cooling then yields a substantially, but not completely, void-free (porosity less than about 5%) coating metallurgically bonded on the substrate. The method for preparing both the hard particle matrix material as well as the braze filler alloy layers or preforms using fibrillated polytretrafluoroethylene (PTFE), is disclosed in U.S. Pat. Nos. 3,916,506 and 4,194,040.
In certain applications, it has been found that even the minimal porosity generated using the foregoing process can be detrimental. Examples of such applications are high pressure seals, articles mandating highly polished surface requirements and/or articles requiring exceptional mechanical properties. One further method of attempting to solve the problem and to increase the desired properties of the coating is to increase the solid density of the matrix layer by using multiple layers of hard particle sheets as disclosed in copending application Ser. No. 475,745, filed Feb. 6, 1990 and assigned to The Pullman Company, assignee of this present application. This method does improve mechanical properties of the coating but does not provide the desired coating in all respects as needed for all types of applications as set forth above.